Fuel cells are an electricity generation system that produces electricity directly by electrochemical reaction of atmospheric oxygen with hydrogen gas or hydrogen obtained by reforming various hydrocarbon fuels (such as natural gases and methane). They provide highly efficient and direct conversion of the fuel's chemical energy to electrical energy. This fact and their non-polluting properties make the generation system more attractive.
The fuel cells are made up of a proton conductive electrolyte membrane (proton conductive membrane) sandwiched between a pair of electrode membranes (fuel electrode and air electrode) on which a catalyst is supported. The catalyst on the fuel electrode separates hydrogen into protons and electrons. The protons pass through the proton conductive membrane and react with oxygen at the air electrode, producing water.
The fuel cells in recent years are required to show higher generating performance. For the fuel cells to generate more electricity, they should be operated at high temperatures. The proton conductive membranes used in the fuel cells are thus required to exhibit high proton conductivity in a variety of environments, in particular at high temperatures.
Polymers having sulfonic acid groups are usually used as proton conductive membranes. The present applicant has proposed proton conductive membranes with high proton conductivity comprising specific polymers having sulfonic acid groups in JP-A-2004-345997 (Patent Document 1), JP-A-2004-346163 (Patent Document 2) and JP-A-2004-346164 (Patent Document 3).    Patent Document 1: JP-A-2004-345997    Patent Document 2: JP-A-2004-346163    Patent Document 3: JP-A-2004-346164